Naturalistic study reveals nuanced cognitive effects of cannabis on frequent older users

A new study published in the Journal of Psychopharmacology suggests that older adults who frequently use cannabis experience specific acute declines in cognitive function, particularly in processing speed and executive functioning. The findings indicate that while these individuals experience strong subjective feelings of intoxication, the impact on their mental performance is nuanced and does not affect all cognitive domains equally.

The demographic of adults over the age of 65 represents the fastest-growing group of cannabis users in the United States and Canada. Despite this rapid increase in prevalence, scientific understanding of how the drug acutely affects the aging brain remains limited. Older adults undergo physiological changes in metabolism and body composition that could theoretically make them more sensitive to the effects of drugs.

On the other hand, many older adults who consume cannabis have done so for decades. This long-term experience might build a tolerance that lessens the cognitive impact of the drug. The research team sought to clarify these uncertainties by observing older adults in a controlled laboratory setting. They aimed to determine how the drug impacts specific mental skills like memory, attention, and problem-solving in a population that is often excluded from clinical trials.

“Since legalization for non-medical purposes in Canada in 2018, use of cannabis is on the rise. This increase is greatest is older adults, and adults over the age of 65 are the fastest growing group of cannabis users. Despite this, there are only a handful of studies on the effects of cannabis in older adults,” said study author Patricia DiCiano, a scientist with the Institute for Mental Health and Policy Research at the Centre for Addiction and Mental Health and an associate professor at the University of Toronto.

To investigate these effects, the researchers recruited 31 adults between the ages of 65 and 78. All participants had normal cognitive function and were current cannabis users. Most reported using cannabis frequently, with many consuming it daily or multiple times a week. The study utilized a naturalistic design to mimic real-world habits.

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Participants were invited to bring their own legally purchased cannabis products to the laboratory. They smoked their preferred amount to reach a desired level of intoxication. This approach contrasts with traditional clinical trials that assign specific, often lower, doses of the drug. It allows scientists to observe the effects of the actual products people use in their daily lives.

One novel aspect of this study is that we invited people to smoke their preferred legally sourced cannabis,” DiCiano said. “Canada has a unique regulatory structure which allows for the testing of retail cannabis in the lab. This lends real world credibility to the findings.”

The experiment followed a counterbalanced, crossover design involving two separate testing sessions. In one session, participants smoked their cannabis. In the other session, they simply relaxed in the same environment without consuming any substances. This structure allowed the researchers to compare the same person in a sober state versus an intoxicated state.

The researchers administered tests measuring different mental skills at 60 minutes and 210 minutes after smoking. These included the Trail Making Test, which measures visual search speed, scanning, speed of processing, and mental flexibility. They also used the Useful Field of View test for visual attention and a verbal free recall task to assess learning and memory.

In addition to cognitive testing, the team collected blood samples to measure levels of delta-9-tetrahydrocannabinol, or THC. They also administered surveys to gauge the participants’ subjective experiences. These surveys asked individuals to rate feelings such as how “high” they felt, their level of sedation, and any changes in mood.

The results showed that smoking cannabis produced robust subjective effects. Participants reported feeling intoxicated and experienced the typical sensations associated with the drug. These feelings peaked around 30 minutes after smoking and remained elevated for hours.

“I think one common misinterpretation is that people who use cannabis frequently will not feel the effects of the drug or the drug will have less of an effect,” DiCiano told PsyPost. “In this study the participants were for the most part frequent users of cannabis and we found that they reported that they felt high, and cannabis had an effect on cognition. All users of cannabis should exercise caution.”

Regarding cognitive performance, the researchers found specific impairments. On the Trail Making Test, participants were slower 60 minutes after smoking compared to their sober session. This test requires connecting numbered dots in order or alternating between numbers and letters. The slower performance suggests a temporary decline in executive function and information processing speed.

The results for memory presented a more complex picture. On the verbal free recall test, participants were read a list of words and asked to repeat them. Later, they were asked to recall as many words as possible. The data showed that the percentage of words retained was higher in the cannabis condition compared to the sober condition.

“We were surprised by the fact that some measures were worse in the no cannabis condition than in the cannabis condition,” DiCiano said. “Some interpret this as an improvement in performance after cannabis, while others posit that cannabis may just reverse fatigue or withdrawal that may become apparent over the course of a day. Participants were for the most part frequent users and these findings would need to be replicated in people who use cannabis less frequently and also with different routes of administration.”

Performance on the Useful Field of View test, which measures the ability to visually attend to central and peripheral information, showed no significant differences between the two conditions. This lack of impairment contrasts with previous studies in younger adults, which often find that cannabis reduces visual attention. It is possible that the older adults in this study had developed a specific tolerance to this effect.

The researchers also analyzed the relationship between blood THC levels and cognitive performance. They found that the correlation was inconsistent and sometimes counterintuitive. For example, higher doses of THC were associated with better performance on a divided attention task at 60 minutes. This contradicts the assumption that higher drug levels always lead to greater impairment.

This study was part of a larger project that also examined driving abilities in the same group of older adults. The companion paper focused on how these same participants performed in a driving simulator. In that portion of the research, the data showed that participants demonstrated increased weaving within their lane and reduced driving speed 30 minutes after smoking.

The cognitive impairments found in the current study, specifically the slowed processing speed on the Trail Making Test, align with the driving difficulties observed. Slower executive functioning can make it harder to react to changes on the road or maintain consistent lane position. The driving study also found that participants reduced their speed, which may be a compensatory strategy to manage their perceived impairment.

“The participants in this study were for the most part frequent users of cannabis and it is important to note that cannabis still had effects on them,” DiCiano explained. “Some people think that frequent users would not be impacted, but the evidence for a diminished impact in frequent users is not well established. Frequent use of cannabis seems to produce a tolerance to some effects of cannabis, but not all.”

Both the cognitive and driving components of the project highlight a disconnect between biological markers and functional ability. Just as blood THC did not predict cognitive decline in a straightforward way, the driving study found no clear link between blood THC concentration and the degree of driving impairment. This suggests that for frequent older users, blood tests may not accurately reflect their level of functional intoxication.

Some limitations affect the generalizability of these findings. The sample size was relatively small and consisted mostly of white males who used cannabis frequently. This makes it difficult to apply the results to occasional users, women, or more diverse populations. The study also relied on participants bringing their own cannabis, meaning the potency and ratio of cannabinoids varied from person to person.

Additionally, the study did not include a placebo group. Participants were aware of when they were consuming the drug and when they were not. This knowledge can influence performance and subjective ratings through expectancy effects. The frequent nature of the participants’ cannabis use also raises the possibility that the “sober” day involved minor withdrawal symptoms, which could affect the comparison.

Future research should aim to compare older adults directly with younger control groups to better understand age-related differences. Scientists also need to investigate other methods of consumption, such as edibles and vaping, which are becoming increasingly popular among older demographics. These methods introduce THC into the body differently and may result in different cognitive effects.

Understanding the chronic versus acute effects of cannabis in this population is also a priority. As more older adults turn to cannabis for medical or recreational purposes, clarity on how it affects their daily functioning is essential for public safety. Large-scale studies could help clarify the complex relationship between tolerance, age, and drug potency.

“Older adults have different cognitive abilities and different body compositions that may affect the way cannabis works; cannabis may produce a greater impact on them,” DiCiano told PsyPost. “On the contrary, they may feel the effects of cannabis less than younger adults due to the fact that they may have more prolonged experience with cannabis. Future research really needs to compare the impact
of cannabis in older adults compared to younger ones.”

The study, “The effects of naturalistic cannabis use on cognition and subjective experience in older adults with normal cognitive function,” was authored by P. Di Ciano, B. Le Foll, S. Zhao, P. Byrne, Y. Elzohairy, J.R. Brubacher, M. McGrath, B. Brands, S. Chen, W. Wang, P. Kaduri, C.M. Wickens, and T.K. Rajji.